Process of eliminating arsenic compounds from phosphoric acid



Oct. 16, 1956 oms ETAL 2,767,136

PROCESS 0. ELIMINATING ARSENIC COMPOUNDS FROM PHOSPHORIC ACID- FiledAug. 31, 1954 Franz Rod/s He/mut Klee INVENTORS m ATTORN Y:

United States PatentO "ice PROCESS OF ELllVHNATlNG ARSENIC COM- POUNDSFROM PHOSPHORIC ACID Franz Rodis and Helmut Klee, Knapsaclr, near Koln,

Germany, assignors to Knapsack-Griesheim Aktiengesellschaft,Knapsack-Bezirk Koln, Germany, a company of Germany Application August31, 1954, Serial No. 453,295-

Claims priority, application Germany September 2, 1953 Claims. (Cl.204130) The present invention relates to a process of eliminatingarsenic compounds from phosphoric acid by electrolysis. For this processthere come into consideration phosphoric acids with a content of about20 percent to about 95 percent of H3PO4, preferably with a contentranging between 50 percent and 90 percent of H3PO4.

The time required for the electro-deposition of arsenic from phosphoricacid containing arsenic compounds is extraordinarily long as compared,under otherwise the same conditions, with the electro-deposition ofother metals having the same place in the electrochemical series. It isknown to use copper as cathode and platinum as anode and a cathodiccurrent density of not more than 1 ampere per square decimetre or toaccelerate the electro-deposition of arsenic from phosphoric acid by thesimultaneous electrodeposition of copped. When using this method ofworking a ratio of the contents of copper and arsenic in the acid of lto at least 2, or better still of 1:25 to 1:4, is advantageously used.Examinations showed, however, that the arsenic concentrations in thephosphoric acid, under otherwise the same conditions, continuouslydecrease with increasing additions of copper ions. Satisfactory resultsare only obtained if about 5 to about times as many copper ions asarsenic are present in the acid.

The present invention is based on the observation that arsenic compoundsare deposited by electrolysis from phosphoric acid by operating withaddition of copper compounds and by depositing the arsenic compounds bythe action of iodine during the electrolysis in the form of arsenic. Itwas found that small amounts of iodine, i. e. a few milligrams perkilogram of acid, increase the speed of deposition of arsenic fromphosphoric acid in an extraordinary and sudden manner. It is ofadvantage to operate at the commencement of the deposition with aproportion by weight of arsenic to copper in the phosphoric acid rangingfrom about lzabout 0.2 to about lzabout 5 as well as with a proportionby weight of copper to iodine ranging from about l00:about 0.5 to about100zabout 50. It has proved of special advantage to maintain theproportion by weight of copper to iodine at a value ranging between100:2 and 100215. The operation is advantageously carried out whilestirring and at temperatures ranging between about 60 C. and about 250C., preferably between 60 C. and about 150 C.

The desired iodine content can be attained by adding an aqueous iodinesolution to the phosphoric acid or by working in such a way that iodineis formed from iodides during the reaction. As iodides there come intoconsideration, in addition to CuI, all soluble iodides such as Nal, Kl,NHI, Cal-z, Balz, SrI2, Mglz. Hydriodic acid may also be used.

As copper compounds are suitable all soluble copper salts either as suchor in solution, the anionic component of which does not have adisturbing effect regarding the purity of the phosphoric acid. There arementioned, for instance, CuCl2, CuSO4, Cu(NO3)z, CuBrz, Cu-acetate.

2,767,136 Patented Oct. 16, 1956 t 2 The use of Cu-phosphate is ofspecial advantage CuI may also be used as was already mentioned.

The examples given further below show that a phosphoric acid which ispractically free from arsenic is obtained when working with an arsenicconcentration of 40 milligrams per kilogram of acid and with an iodineconcentration of about 1.8 milligrams per kilogram of acid. The copperions are deposited on the cathode and are no longer detectable in thephosphoric acid after the electrolysis.

When copper ions are added in the form of copper sulfate, merely thesulfate anion of the copper sulphate remains in the phosphoric acid. Ifone wants to avoid this, copper phosphate must be added straight away orthe necessary quantity of copper must be dissolved in the requiredquantity of phosphoric acid at the anode.

The addition of iodine should advantageously be kept in its limits sinceotherwisethe elimination of arsenic is deteriorated as is shown by. thefollowing table but Addition to phosphoric Content of acid arsenic inthe phosphoric Cu ions, Iodine, acid, mgsJkg. of mgsJkg. mgsJkg.

acid of acid of acid untreated phosphoric 40 7 acid 40 0. 6 3 40 1. 2 140 1. 8 0 40 3.0 6

From the above table can be seen that, when reducing the addition ofiodine from 1.8 to 0.6 milligrams per kilogram of acid, the content ofarsenic rises again from practically zero to 3 milligrams per kilogramof acid. When raising the addition of iodine from 1.8 to 3 milligramsper kilogram of acid the content of arsenic in the phosphoric acid riseslikewise from practically zero to 5milligrams per kilogram of acid.

An increase of the copper ions is likewise detrimental. Using a contentof 1.8 milligrams of iodine per. kilogram of acid and increasing theadditions of copper ions from 40 to 68 and then to 112 milligrams, thearsenic content rises from practically zero to 3 milligrams per kilogramof phosphoric acid.

The process according to the present invention is illustrated in'theaccompanying diagrammatic drawings. Figure 1 represents a side elevationof the apparatus, Figure 2 represents a top view.

140 milligrams of copper ions per kilogram of acid in the form of a veryconcentrated solution of copper salt and l.8,millig1ams' of iodine perkilogram of acid-in the form of an aqueous 0.1N-solution of iodine areadded to the phosphoric acid which contains arsenic. The whole is passedthrough the tube 4 into a double-walled vessel 3 which has an acid-proofbrick lining and which is provided with a supply pipe 1 and a drain pipe2 for the heating liquid and is then heated to about C. by means of theheating liquid. The electric current required for the electrolysis issupplied over the anode 7 having a cylindrical shape and consisting of aplatinum wire gauze and over the cathode 8 being likewise of acylindrical shape and consisting of a copper wire gauze. The stirrer 6efiects the thorough mixing of the reaction mixture. On termination ofthe electrolysis which takes place under a tension of about 3 volts andat an amperage of about 5 amperes, the phosphoric acid which is now freefrom arsenic leaves the electrolytic vessel 3 through the outlet 5.Supply and discharge of the phosphoric aye-arse acidare regulated insuch away that the acid is subjected to electrolysis tor about 30minutes.

The process may also be carried out discontinuously. The followingexamples serve to illustrate the inventio but they'are' notintended tolimit it' theretoi Emmnle 1 3 litres of phosphoric acid (8.085,percent), which contain 40 milligrams of arsensic per kilogram of acidin the form of arsenic compounds, are heatedto 9.0? C. and stirred bymeans of, a stirrer (3-5 revolutions per second). Into this phosphoricacid are placed as cathode two copper wire gauzes of cylindrical shapewhich are inserted into each other, and have a height of 13 cuts. 2 -2 dameter ofi 4 and. BI cms. respectively. Between the ZIcopper wiregauzesai platinum. wire-gauze electrode is placed as. anode. '40.milligrams of copper ions per kilogram of acid in the form of an aqueouscopper sulfate. sohltion-as concentrated as possible and 1.8 milligramsof iodine per kilogram. of acid injthe form of an aqueous 0.1 N-solutionof iodine 'are'added and. the acid is electrolyzed under 3. volts. andat 5 'amperes." After about 30. minutes the acid'is free from arsenic.'

. rmi e 3 litres of phosphoric acid (SO-8.5 percent) which contain 40milligrams of arsenic per kilogram of acid in the form of arseniccompounds are heated to 90 C. and stirred by means "offa stirrer,arrangement of the electrodes is' the same, as described in Example 1.40 milligrams of copper ions per'ki-logram of acid in the form of anaqueous copper-sulfate. solution as concentrated as possible and 1.8milligrams of iodine per kilogram of acid in the form of an aqueous 0.1N-po- 'tassium or sodium iodide solution are added and the acid iselectrolyzed wide the sane q ndi pns described in Example 1. Aphosphoric acid which is free from arsenic sobta n d Emmy? 3 3: litresof phosphoric acid (-80-85percent) which contain- 4.0v milligrams 'ofiarsenic "per "kilo ram of 'a'cid in the form of arsenic compoundsareheated-Q90 C. and stirred. by means. of a stirrer. The afrangement oftheelectrodes is thesame as described 1 40 milligrams of copperpe'rkilogranioff-acid e form of! dry. copper phosphate and"l.'8' milligramsof iodine per kilogram of-acid in 'the form of an aqueous 0L1" N-iodine: solution are then added and the acidis eleotrolyzed :as.described in Example 1. 'A phbsphorlc acid which is "free from: arsenicis obtained; i

B t- 1 13. 4 3 litres of phosphoric acid (.80 '-85-percent)-whichcontain 4,Q milligrams. of arsenic per: kilogram of acid in the form ofarsenic compounds are heated; to,90- crane stirred: by meansf of astirrer. The arrangement ofthe electrodes. is the same as'de'scribed inExample 1-." 40 of copper per. kilogram" ofi acid in the form of drycopper phosphate and 1.8 milligrams of iodine per kilogram of acid inthe form of an aqueous 0.1 N-solution of potassium or sodium iodidesolution are added and the acid is electrolyzed under the sameconditions as described in l jigample l. A phosphoric acid which is freefrom arsenic i's obtained.

We claim: a

P QI o l mina i arse i ompoun from phosphoric acid by electrolysis, inwhich .the electrolysis is'oarried out with a phosphoric acid which, atthe commenee'ment r the deposition, contains arsenic. and copper' in aproportion by weight ranging from between about lziabout 0.2 to aboutl:about 5,' and copper and iodine in a proportion by. weight ranging.from between about 4010,6to 40 23.0.

2.A process of eliminating arsenic compounds from phosphoric acid byelectrolysis, in which the electrolysis is carried out with a phosphoricacid which contains a soluble copper salt in such a quantity that, atthe commencement ofthe deposition, the proportion by weight ofarsenic tocopper. ranges from between about 1:about 0.2 tov about liabout 5, and.which contains iodine in such a quantity that the propbrtionby weight.of copper to iodine ranges from between 40:0.6 to 4013.0.

3. A'procss. of eliminating arsenic compounds from phosphoric acid. byelectrolysis, in which the electrolysis is. carried out with aphosphoric acid which, at the commencement of the deposition, containsarsenic and copper in'a proportion by weight ranging from between aboutl z-about 0.2 to about lzabout 5, and in which an iodine solution is.added to-the phosphoric acid insuch a quantity. that the proportion byweight of copper to iodine ranges from. between about 40:06 to 40:30.

4; A process ofeliminating-arsenic compounds from phosphoric acid byelectrolysis, in which the electrolysis I iscarried' out with aphosphoric acid which, at the com-' mencement of the deposition,contains arsenic and copper in a proportion by weightranging frombetween about l:'about0.2'to about lfabout 5 and in which the desirediodine content is attained by 'the formation of iodine "front iodidesduring the reaction, these iodides being added to the phosphoric :acidin such a quantity that copper and iodine are present in a proportion byweight ran'ging from betweenabout 40-:0.-6 to 40:3.0.

SPA process of eliminating arsenic compounds from phosphoric acid byelectrolysis, in which the electrolysis is'carried out with a phosphoricacid which, at the com= mencement of the deposition, contains arsenicand: copper'in a proportion by weight ranging from between :about1-:about 0.Z to about l za'bout 5, and in which the desired iodinecontentis attained by the formation of iodine firom hydriodic acidduring the reaction, the hydriodie acid being -addedto thephosphoricacid in such a quantity that'copperand ia'dirie are presentin; a proportionby weight ranging rmmbetween 4020.6 to 4023.0. f

' No references cited.

1. A PROCESS OF ELIMINATING ARSENIC COMPOUNDS FROM PHOSPHORIC ACID BYELECTROLYSIS, IN WHICH THE ELECTROLYSIS IS CARRIED OUT WITH A PHOSPHORICACID WHICH, AT THE COMMENCEMENT OF THE DEPOSITION, CONTAINS ARSENIC ANDCOPPER IN A PROPORTION BY WEIGHT RANGING FROM BETWEEN ABOUT 1:ABOUT 0.2TO ABOUT 1:ABOUT 5, AND COPPER AND IODINE IN A PROPORTION BY WEIGHTRANGING FROM BETWEEN ABOUT 40:0.6 TO 40:3.0.